Such a diffuser is described in British Pat. No. 1,024,328.
In said prior diffuser, the fluid flows along the outer wall in the general inlet-to-outlet direction over the portion of the wall upstream from the bleed slot, but the flow downstream from the slot along said outer wall is reversed, i.e. it is from the outlet towards the bleed slot.
This causes separation downstream from the slot.
Preferred embodiments of the present invention enable such separation to be avoided.